In certain analog signal processing applications, capacitors are added to a signal path to adjust the phase of the signal. This may be performed to match the phase of the signal relative to another signal, or it may be performed to tune the filter characteristics, for example. One of the simplest and the most straight-forward techniques to control the capacitance to be adjusted is by connecting a metal oxide semiconductor (MOS) switch in series with the capacitor. Thus, circuitry can turn on the switch to add capacitance to a given signal processing circuit, and turn off the switch to not add the capacitance.
Generally, the switch size is selected large enough such that the on-resistance of the switch is small. For a signal frequency that is 100 MHz or less, for example, selecting a very large switch has little side effect to the signal linearity. However, when the signal frequency goes into several hundred MHz, for example, using a large switch can have a negative effect on the signal linearity when the switch is turned off.